VCSELs are commonly used as light sources in optical transceivers for local area networks (LANs) and storage area networks (SANs). Oxide-apertured VCSELs are currently the dominant technology for such applications. However, a mesa VCSEL was recently developed that includes a mesa, rather than an oxide aperture, for current confinement, as disclosed in U.S. Pat. No. 7,095,768, issued on Aug. 22, 2006 to Jiang, et al., which is owned by the assignee of the present invention and is incorporated herein by reference. Advantageously, the mesa VCSEL is both more reliable and more easily manufactured than oxide-apertured VCSELs. Unfortunately, the mesa VCSEL has the disadvantage of a somewhat higher level of relative-intensity noise (RIN) in relation to oxide-apertured VCSELs. At data rates above 4 Gbit/s, the RIN of the mesa VCSEL can reach an unacceptable level.
With reference to FIG. 1, such a prior-art mesa VCSEL 100 includes a contact layer 110 of metallic material, a substrate 120 of semiconductor material, a distributed Bragg reflector (DBR) layer 130 of semiconductor material, an active layer 140 of semiconductor material, a DBR mesa 150 of semiconductor material, a planarization layer 160 of dielectric material, and a contact annulus 170 of metallic material. In particular, the active layer 140 is disposed on a top surface of the DBR layer 130, and the DBR mesa 150 is disposed on a mesa region 141 of a top surface of the active layer 140. The contact annulus 170 is disposed on a contact region 151 of a top surface of the DBR mesa 150, such that an inner circumference of the contact annulus 170 defines a window region 152 of the top surface of the DBR mesa 150.
According to experiments, the level of RIN of the mesa VCSEL 100 increases as the inner circumference of the contact annulus 170 decreases, for a constant outer circumference of the contact annulus 170. Thus, the somewhat higher level of RIN of the mesa VCSEL 100 may be a result of lasing sustained in the active layer 140 under the contact annulus 170. As indicated by arrows in FIG. 1, light resonates in the active layer 140 under the DBR mesa 150, but is only emitted from the window region 152 of the top surface of the DBR mesa 150. As the mesa VCSEL 100 lases in several transverse modes which have different fractions of their optical power under the contact annulus 170, the optical power emitted from the mesa VCSEL 100 varies slightly, but randomly, over time, owing to competition between the transverse modes.
An object of the present invention is to overcome the shortcomings of the prior-art mesa VCSEL 100 by providing an improved mesa VCSEL, in which lasing is sustained in the active layer mainly under the window region.